Problem with array subscripts in plpgsql trigger function

Ok, I have a trigger set up on the following (stripped down) table:

CREATE TABLE members (
member_id bigint,
member_status_id smallint,
member_is_deleted boolean
);

Here's a shortened version of the trigger function:

CREATE OR REPLACE FUNCTION update_member() RETURNS TRIGGER AS $um$
DECLARE
status_deltas integer[];
BEGIN
IF(NEW.member_status_id != OLD.member_status_id AND
NEW.member_is_deleted IS NOT TRUE) THEN
status_deltas[NEW.member_status_id] := 1;
status_deltas[OLD.member_status_id] := -1;
END IF;
/*and after a couple more such conditional assignments I use the
values in status_deltas to update another table holding status totals here*/
END;
$um$ LANGUAGE plpgsql;

on the two lines that access set array values I'm getting the following
error:

ERROR: invalid array subscripts

What gives?

--
erik jones <erik(at)myemma(dot)com>
software development
emma(r)